Here is an excerpt from Christine Delsol's article in the San Francisco Chronicle:
"The first question a traveler asks these days when considering a trip to Mexico is not the time-honored "How do I avoid getting turista?" or "How many pesos will I get for my dollar?" but "Is it safe?" Until recently, the answer was easy: Yes, if you keep informed and stick with the popular tourist destinations. In the past year, that has begun to change. Highly publicized drug-cartel killings in Acapulco's tourist zone, a hand grenade in a popular Guadalajara nightclub only a block from the upscale Hotel Fiesta Americana, and muggings and shootings near Mazatlán's cruise port are signs that tourist destinations are no longer insulated from the drug-related violence wracking Mexico. Monterrey, one of Mexico's most important medical tourism destinations and an upscale city that enjoyed a quiet distance from the brunt of the conflagration in its early years, has seen a raft of shootouts and kidnappings, prompting the State Department to order all its personnel in the city to remove their children from the city. In Guadalajara, U.S. government officials are prohibited from traveling after dark.
"It's not a sudden change of heart among the drug lords, but a perhaps inevitable progression of turmoil within the country's drug-trafficking network that has been simmering for more than a decade. Contrary to one popular misconception, the cartels have never given tourists a free pass because of the money they bring in, or for any other reason — tourists simply weren't relevant to the drug business. Nor are tourists relevant now. If cartels do not have a hands-off policy, neither do they have any particular interest in going after tourists. Rival gangs are their main focus, with law enforcement authorities, government officials and journalists close behind. Visitors are not the targets of the violence, but rather we are closer to the violence in certain places, which is unnerving at best and dangerous at worst. ut if it feels like all bets are off and random violence is everywhere, that's not completely true, either. There are reasons the violence erupted when and where it did, and it's surfacing in tourist areas now for specific reasons. This context, absent from most reports about the latest killing or kidnapping, might not be much comfort, but it at least provides an informed basis for deciding whether and where to travel." Link to Full Article
Analysis: I really don't have much more to add, but I did want to let my readers know that this is one of the best explanations I've seen regarding how and why drug-related violence has moved into some tourist areas. There is more good info in the article, but it's lengthy; I highly recommend that you follow the link and read the whole thing.
The main point Ms. Delsol makes is that the DTOs aren't moving into places like Acapulco and Mázatlan because of the tourists, but in spite of them. And it's true when she says this is both good and bad news. It's good because it means that tourists aren't in DTO crosshairs. However, the violence is going to shift geographically based on how the supply chain adapts to business and enforcement conditions. Sometimes that's unpredictable, and kidnappings and shootouts can start erupting in once-quiet areas in a matter of days.
Bottom line, know before you go, as cheesy as that sounds. Do your homework, contact the US Consulate closest to the area where you'll be traveling, and do a Google News search for that city or town to see if there have been any incidents there related to the drug trade.